1. Field of the Invention
The present invention relates to an oxygen concentration sensor for the exhaust gas of an engine. More specifically, the present invention relates to:
detecting the impedance (that is, the alternating-current impedance) or the admittance of the oxygen concentration sensor; PA1 an oxygen-concentration detecting apparatus which detects the resistance of the oxygen concentration sensor; and PA1 detecting the temperature of the oxygen concentration sensor according to its resistance component. PA1 an output circuit for outputting a voltage supplied to an input terminal of the output circuit to the oxygen concentration sensor; PA1 a constant-voltage circuit for supplying a first predetermined constant voltage, to the input terminal of the output circuit through an output resistor; and PA1 a voltage switching circuit having a voltage dividing resistor and a switching device, which are connected in series between a signal line from the output resistor to the input terminal of the output circuit and a first terminal having a second predetermined voltage.
2. Description of Related Art
In recent years, there has been a demand for an increased control accuracy and a demand for a change to lean burning in control of the air-fuel ratio of an engine used in a vehicle. In response to such demands, there has been provided a linear air-fuel-ratio sensor or an oxygen concentration sensor capable of detecting the air-fuel ratio of mixed air supplied to the engine or the concentration of oxygen contained in exhausted gas which varies with the current flowing through sensor linearly over a wide range.
In order to maintain a high detection accuracy of such an air-fuel-ratio sensor, it is necessary to keep the sensor in an activated state. In general, in order to sustain the activated state of the air-fuel-ratio sensor, it is necessary to heat the sensor element by controlling the current supply to a heater attached to the sensor.
Regarding such current-supply control to the heater, there has been developed a conventional technology of implementing feedback control whereby the temperature of the sensor element is detected and adjusted to a desired activation temperature of typically about 700 degrees Celsius(.degree. C.). The temperature of the sensor element is referred to simply as an element temperature. As a possible technique to detect the element temperature from time to time, a temperature sensor may be attached to the sensor element. The element temperature is then derived from output by the temperature sensor. With such a technique, however, the sensor will become large in size and its cost will rise.
In order to solve the problem described above, there has been proposed a technique whereby the impedance of the sensor element which is also referred to hereafter simply as an element impedance is detected instead of the element temperature. The element temperature can then be found from the detected element impedance by using a known relation between the element temperature and the element impedance. It should be noted that a result of detection of the element impedance can also be used for, among other purposes, determining the degree of deterioration of the air-fuel-ratio sensor.
With the conventional element-impedance detecting apparatus, however, the element impedance can not be detected accurately in some cases.
Furthermore, as a technique of detecting element impedance of an air-fuel-ratio sensor of the limit-current type for example, a voltage Vneg is applied to a resistance-dominant zone not including a limit-current zone, and a current Ineg flowing through the sensor as a result of the application of the voltage Vneg is measured. The element impedance is then found as a ratio of the voltage Vneg to the current Ineg as follows: EQU Element impedance=Vneg/Ineg
According to another technique of detecting an element impedance, an applied voltage is lowered or raised, and a decrease or an increase in flowing current resulting from the decrease or the raise in applied voltage is measured. The element impedance (alternating-current element impedance), is determined by the decrease or increase in applied voltage and the decrease or increase in the flowing current.
According to the conventional technologies, however, conversion from an element impedance to an element temperature may cause an error, and the detection accuracy of the element temperature may be compromised. This problem of a poor detection accuracy is explained as follows. A relation between the element impedance and the element temperature shown in FIG. 25 is known. The vertical axis of FIG. 25 represents the impedance count value obtained as a result of LSB conversion of a detected element impedance. As shown in the figure, the impedance count value is not inversely proportional to the element temperature. Particularly, in the zone of low element temperatures, the impedance count value increases abruptly as the element temperature decreases.
In the activation-temperature zone, on the other hand, a difference of 1 count in impedance count value corresponds to a large difference in element temperature. In an activation-temperature zone of the sensor element, for example, the following relations between the impedance count value and the element temperature exist. When the impedance count value changes from 26 to 25, the derived element temperature substantially changes from 746 degrees Celsius to 750 degrees Celsius. Likewise, when the impedance count value changes from 25 to 24, the derived element temperature greatly changes from 750 degrees Celsius to 760 degrees Celsius. Accordingly, such large change in the element temperature causes a problem that the element temperature is not detected accurately.